Photomask having a plurality of shielding layers

ABSTRACT

Some embodiments pertain to a photomask for mask patterning. The photomask includes a phase shift layer overlying a transparent layer, a first shielding layer overlying the phase shift layer, and a second shielding layer overlying the first shielding layer. The first shielding layer has a first optical density, and the second shielding layer has a second optical density. The second optical density is less than the first optical density.

REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No.16/153,935, filed on Oct. 8, 2018, which is a Continuation of U.S.application Ser. No. 15/362,089 filed on Nov. 28, 2016 (now U.S. Pat.No. 10,095,102, issued Oct. 9, 2018), which claims the benefit of U.S.Provisional Application No. 62/321,448, filed on Apr. 12, 2016. Thecontents of the above-referenced Patent Applications are herebyincorporated by reference in their entirety.

BACKGROUND

Integrated circuits (ICs) are manufactured by transferring geometricpatterns from photomasks to light-sensitive material known as“photoresist”. In particular, a geometric pattern is formed on a layerof photoresist on a semiconductor substrate by providing light through aphotomask. The photomask includes a transparent layer that is partiallycovered with an opaque material. The portions of the transparent layercovered with opaque material block light, while the remaining uncoveredportions of the transparent substrate allow light to pass there through,such that the light passing through the photomask transfers a pattern tothe photoresist. After the photoresist has been exposed in this fashion,the photoresist is developed to selectively remove portions of thephotoresist which were exposed to (or not exposed to) light, dependingon whether the photoresist has a negative or positive tone. In someinstances, the underlying wafer can then be etched with the patternedphotoresist in place and the photoresist layer can be subsequentlyremoved. Multiple patterned layers can be built up on the IC in thisfashion to fabricate overall IC design.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a photomask having multipleshielding layers.

FIG. 2A illustrates one embodiment of a patterned wafer corresponding tothe photomask shown in FIG. 1.

FIG. 2B illustrates another embodiment of a patterned wafercorresponding to the photomask shown in FIG. 1.

FIG. 3 illustrates an embodiment of a photomask stack having multipleshielding layers for mask patterning.

FIGS. 4A-4D illustrate embodiments of a photomask stack being etched toform the patterned photomask having multiple shielding layers.

FIGS. 5-15 illustrate a series of cross-sectional views of someembodiments of a photomask at various stages of formation, the photomaskstack and the patterned photomask having a plurality of shieldinglayers.

FIG. 16 illustrates a flowchart of some embodiments of a method forforming a photomask stack and subsequent photomask having a pluralityshielding layers.

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples,for implementing different features of this disclosure. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In response to increasing miniaturization of integrated circuits (IC),device structures are more densely arranged on a substrate. The densearrangement of device structures makes precision and accuracy in theformation and arrangement of the device structures increasinglyimportant. Typically, the device structures are formed on the substrateusing photolithography. In photolithography, light is used to transfer apattern from a photomask to a photoresist on the substrate. The lightused to transfer the pattern is a function of optical density.

Optical density is the logarithmic ratio of the intensity of transmittedlight to the intensity of the incident light passing through substancematerial. In particular, the optical density of a material relates tothe sluggish tendency of the atoms of the material to maintain theabsorbed energy of an electromagnetic wave in the form of vibratingelectrons before reemitting it as a new electromagnetic disturbance. Themore optically dense that a material is, the slower that a wave willmove through the material. In some embodiments, optical density ismeasured as the absorbed radiation of the corresponding wavelength. Oneindicator of the optical density of a material is the index ofrefraction value of the material. Accordingly, optical density can referto the absorbance of a particular layer, a combination of layers, or astructure.

To achieve miniaturization, finer photoresist patterns are used to formdevice structures. However, the thickness of the photomask and/orphotoresist can cause light scattering that results in undesirablepattern variation. Accordingly, reduction in the thickness of thephotomask and/or photoresist can improve pattern fidelity. Furthermore,creating a photomask corresponding to the desired pattern includesmultiple steps of layering and etching. The more complex this process,the more likely the complexity is to induce loading effects, such as,blurring, rounding, and shortening of the pattern. Accordingly,simplifying the process of creating the patterned photomask would alsoimprove pattern fidelity.

Here, a photomask having a plurality of shielding layers is described.In some embodiments the photomask includes a phase shift layer overlyingthe transparent layer and at least two shielding layers overlying thephase shift layer. The thickness and composition of a shielding layercan be tuned to achieve a particular optical density for that shieldinglayer. Because the shielding layer is formed to block the transmissionof light, a specific optical density may be desired to achieve apattern. Because at least two layers are used, the thickness of theshielding layer is not based solely on the optical density of a singlelayer, but rather the optical density of the combined shielding layers.

For example, typically chromium is used as a shielding layer. However,to achieve a desired optical density of 1.8, a chromium shielding layerwould have to be 52 nanometers (nm) thick, which is relatively thick andcan cause fidelity issues. To reduce these fidelity issues, at least twoshielding layers with different thicknesses and different opticaldensities from one another can be used to achieve a multi-layershielding layer with the same optical density as the 52 nm chromiumshielding layer but with a thinner overall profile than 52 nm. Forexample, a first shielding layer may be 5 nm thick and have an opticaldensity of 0.23, while a second shielding layer may be 24 nm thick andhave an optical density of 1.6. Accordingly, the combined opticaldensity of the pair of shielding layers is greater than 1.8 with acombined thickness of 29 nm instead of the larger 52 nm single shieldinglayer of chromium. This may reduce the overall size of the patternedphotomask while increasing the overall optical density, such as to 3.0.Thereby, making the patterned photomask thinner with a higher opticaldensity, and thus, improving pattern fidelity.

Furthermore, the composition of the at least two shielding layers can beselected to simplify the process of creating the photomask in order toreduce loading effects. Suppose that a first shielding layer overliesthe phase shift layer, and a second shielding layer overlies the firstshielding layer. The materials for the first and second shielding layersmay be selected so that both the first and second shielding layer can beetched by one etchant. Likewise, the material of the first shieldinglayer may be selected so that a single etchant can etch both the firstshielding layer and the phase shift layer. Thus, the first shieldinglayer does not require its own separate etching step. In this manner,the process for creating the photomask can be simplified, therebyfurther improving the pattern fidelity.

FIG. 1 illustrates some embodiments of a mask patterning system 100having a mask writing tool 102 configured to pattern a patternedphotomask stack 104. The patterned photomask stack 104 overlies atransparent layer 106, thereby generating a photomask. The mask writingtool 102 is configured to produce a pattern in a photoresist layer (notshown). The patterned photomask stack 104 includes sections of a phaseshift layer 108, a first shielding layer 110, and a second shieldinglayer 112. The pattern in the photoresist layer is transferred bystrategically etching the phase shift layer 108, the first shieldinglayer 110, and the second shielding layer 112 to create patternedphotomask stack 104. In various embodiments, the patterned photomaskstack 104 may be a binary intensity mask (BIM) or attenuated phase shiftmask (APSM). Once the patterned photomask stack 104 is etched, thephotoresist can be removed.

Strategic etching of the patterned photomask stack 104 into layeredsections allows the photomask to develop a desired pattern. In someregions, such as 114 a, 114 b, and 114 c the incident radiation isblocked by sections of the first shielding layer 110 and the secondshielding layer 112. The first and second shielding layers 110, 112 areopaque layers that block the transmission of electromagnetic radiationthrough the transparent layer 106. The light blocking function of thefirst and second shielding layers 110, 112 can be accomplished eventhough the first shielding layer 110 and the second shielding layer 112have a combined thickness less than that of a typical shielding layerdue to the combined optical density of the first and second shieldinglayers 110, 112.

When incident radiation is able to strike the transparent layer 106, forexample at regions 116 a, 116 b, 116 c, and 116 d, the incidentradiation causes a pattern to be formed on a wafer, as will be discussedwith respect to FIGS. 2A and 2B. In other regions, such as regions 118 aand 118 b, the incident radiation passes through the phase shiftinglayer 108 to the transparent layer 106. The phase shift layer 108 isconfigured to allow a small amount of radiation to be transmittedthrough (typically just a few percent). That radiation is not strongenough to create a pattern in and of itself. Instead, the radiationpassing through the phase shift layer 108 causes interference in theradiation coming through the surrounding regions of transparent layer106 to improve pattern fidelity.

The regions 114 a-c, 116 a-d, and 118 a-b of the patterned photomaskstack 104, are formed such that together the regions cause a pattern tobe formed on the wafer when the radiation passes through the patternedphotomask stack 104 to a wafer.

FIG. 2A illustrates an example embodiment of wafer 200A that wouldtheoretically result from use of the photomask described with respect toFIG. 1. For example, portions of the wafer 200A underlying regions 114a, 114 b, and 114 c do not result in the wafer being patterned. Whileportions of the wafer 210A underlying regions 116 a, 116 b, 116 c, and116 d are patterned as exemplified by patterned portions 200 a, 210 b,210 c, and 210 d of the wafer 200A. As described above, even thoughincident radiation at regions 118 a and 118 b does not pass through thefirst shielding layer 110 and the second shielding layer 112 (shown inFIG. 1), the phase shift layer 108 (shown in FIG. 1) only allows a smallpercentage of light to pass through which does not result in patterning.Accordingly, portions of the wafer 200A underlying regions 118 a and 118b are not patterned.

FIG. 2B illustrates an alternative embodiment of wafer 200B that wouldtheoretically result from use of the photomask described with respectwith FIG. 1 depending on the three dimensional nature of the patternedphotomask stack 104. For example, portions of the three-dimensionalwafer 200B underlying regions 214 a, 214 b, 214 c, and 214 d results inthe wafer being patterned in a limited manner underlying those areas. Inthis embodiment, the wafer 200B regions underlying 216 a, 216 b, 216 c,216 d, and 216 e may patterned in a less limited manner than the regionsunderlying 214 a, 214 b, 214 c, and 214 d because the patternedphotomask stack 104 does not overlie the regions 216 a, 216 b, 216 c,216 d, and 216 e. Likewise, regions underlying 218 a and 218 b arepatterned more than regions underlying 214 a, 214 b, 214 c, and 214 dand less than regions underlying 216 a, 216 b, 216 c, 216 d, and 216 edue to the phase shift layer 108.

Using the patterned photomask stack 104, the critical dimension (i.e.,the minimum feature size) can be improved. For example, criticaldimension uniformity can be generally improved by 20% using the phaseshift layer 108, the first shielding layer 110, and the second shieldinglayer 112 of the patterned photomask stack 104. However, the criticaldimension may be improved even more depending on the specificapplication. For example, in one application, the patterned photomaskstack 104 is used to form a static random-access memory (SRAM) chip on awafer, such as the wafer 200A or 200B. The critical dimension uniformityfor an SRAM, patterned using the patterned photomask stack 104, can beimproved by 50%.

Moreover, the two-dimensional (2D) fidelity is also improved. Forexample, a mean critical dimension can be calculated by determining thedifference between the actual critical dimension of a feature and thetarget critical dimension of the feature. In the embodiment of an SRAM,as discussed above, the SRAM mean is calculated by taking the differencebetween an actual SRAM critical dimension from the target SRAM criticaldimension. This is done in 2D because the measurement is taken in afirst dimension corresponding to the vertical/horizontal pattern, and asecond dimension corresponding to the area critical dimension. Both ofthese are used to demonstrate the improved 2D fidelity. Using thepatterned photomask stack 104, the 2D fidelity can be improved by 30%.

FIG. 3 illustrates an embodiment of a photomask stack 300 havingmultiple shielding layers for mask patterning. The photomask stack 302overlies the transparent layer 106. A photoresist 304 overlies thephotomask stack 302. The photoresist 304 is etched such that thephotomask stack 302 can subsequently be etched into the sections asillustrated by the photomask of FIG. 1. The resulting photomask can beused is used in lithography to pattern a wafer.

The photomask stack 302 includes contiguous layers of the phase shiftlayer 108, the first shielding layer 110, and the second shielding layer112. The phase shift layer 108 overlies the transparent layer 106. Insome embodiments, the phase shift layer 108 has a thickness in a rangeof about 60 nm to about 80 nm and is comprised of a phase shiftmaterial. The first shielding layer 110 and the second shielding layer112 are configured to block the transmission of electromagneticradiation. Accordingly, the first shielding layer 110 and the secondshielding layer 112 are opaque.

The parameters (e.g., thickness, material, etc.) of the first shieldinglayer 110 and the second shielding layer 112 are tunable such that theproperties (e.g., susceptibility to etchant, optical properties, etc.)of the first shielding layer 110 and the second shielding layer 112 canbe selected. Tuning the parameters of the first shielding layer 110 andthe second shielding layer 112 may also simplify the patterning process.

Suppose, the first shielding layer 110 is comprised of a first materialand the second shielding layer 112 is comprised of a second material. Insome embodiments, the first material and the second material havedifferent chemical compositions. For example, the shielding layers maybe comprised of different transition metals. In one embodiment, thefirst material may include a group five transition metal, such atantalum, and the second material may include a group six transitionmetal, such as chromium. The first material and the second material maybe selected so that both the first shielding layer 110 and the secondshielding layer 112 can be etched by the same etchant, a first etchant.Accordingly, the first shielding layer 110 and the second shieldinglayer 112 can be etched in a single step rather than individual etchingsteps per layer. To further simplify the process, the first material maybe selected such that the phase shift material of the phase shift layer108 and the first material can be etched together using a secondetchant.

Additionally or alternatively to selecting materials based on theiretching properties, the materials may be selected based on their opticalproperties. The optical properties may be based on the thickness of thelayer. In one embodiment, the first shielding layer 110 has firstthickness of approximately 20 nm to 28 nm. In another embodiment, thesecond shielding layer 112 has a second thickness of approximately 3 nmto 7 nm. The materials or properties of the first shielding layer or thesecond shielding layer may also be selected based on the materials orproperties of the other.

For example, the first material of the first shielding layer 110 may beselected based on the second shielding layer 112 being 5 nm thick andhaving an optical density of 0.23. In one embodiment, the firstshielding layer 110 may be selected for having an optical density of 1.6when 24 nm thick, which would achieve a desired optical density ofgreater than 1.8 when the optical densities of the first shielding layer110 and the second shielding layer 112 are combined.

Accordingly, the combined optical density of the first shielding layer110 and the second shielding layer 112 is greater than 1.8 with acombined thickness of 29 nm, instead of the typical, larger, 52 nmsingle shielding layer of chromium.

FIG. 4A illustrates an embodiment of a photomask stack having multipleshielding layers for mask patterning. The photomask stack 400 includesthe layers described above with respect to FIG. 3 and the layersfunction in a similar manner. For example, the photomask stack 400includes continuous layers of the transparent layer 106, the phase shiftlayer 108, the first shielding layer 110, the second shielding layer112, and the photoresist 304. Here the layers are continuous because thephotomask stack 400 is illustrated before etching has occurred.

FIG. 4B illustrates one embodiment of a photomask stack 402 in which thephotoresist 304 has been etched to expose a portion of the secondshielding layer 112. The photoresist 304 is etched based on desiredpattern of the resulting patterned photomask, as shown by the photomaskof FIG. 1. The photoresist 304 prevents areas of the underlying secondshielding layer 112 from being etched. However, exposed portions of thesecond shielding layer 112 will be able to be etched.

FIG. 4C illustrates one embodiment of the first shielding layer 110 andthe second shielding layer 112 having been etched. As discussed abovewith respect to FIGS. 2A and 2B, the first shielding layer 110 and thesecond shielding layer 112 may be etched by a single etchant in oneprocess step. Accordingly, when the second shielding layer 112 is etchedat least a portion of the first shielding layer 110 underlying thesecond shielding layer 112 is also etched.

Suppose that the first shielding layer 110 has a thickness, t₁, thesecond shielding layer 112 has a thickness t₂, and that the firstshielding layer 110 and the second shielding layer 112 are etched with afirst etchant. The first etchant may be include a chemical compositionof chlorine and oxygen. In some embodiments, the first etchant etchesthe entire thickness t₂, of the second shielding layer 112 at portionsof the second layer exposed by the photoresist 304, as shown above inFIG. 4B. Thus, the first etchant etches the second shielding layer 112into sections. In particular, the second shielding layer 112 isseparated into a plurality of second layer shielding sections separatedby gaps.

Furthermore, portions of the first shielding layer 110 underlying thegaps in the second shielding layer 112 are also etched to form aplurality of first layer shielding sections. For example, the gaps ofthe second shielding layer 112 have a first sidewall 404 and a secondsidewall 406. Because the first etchant partially etches the firstshielding layer 110 too, there are sections of the first shielding layerthat are also separated by gaps. The sidewalls of the gaps in the firstshielding layer 110 are aligned with the first sidewall 404 and thesecond sidewall 406 of the second shielding layer 112. In this manner,the first etching step forms a plurality of shielding sections in theshielding layers.

The first sidewall 404 of the second shielding layer 112 is aligned withthe first sidewall 408 of the first shielding layer 110. Likewise, thesecond sidewall 406 of the first shielding layer 110 is aligned with thesecond sidewall 410 of the second shielding layer 112. In someembodiments, the first sidewall 408 and the second sidewall 410 of thefirst shielding layer 110 may not extend through the thickness, t₁.Instead, the first sidewall 408 and the second sidewall 410 of the firstshielding layer 110 may extend to a first depth, d₁, that is less thanthe thickness, t₁, of the first shielding layer 110.

FIG. 4D illustrates a patterned photomask 412 having etched phase shiftlayer 108, an etched first shielding layer 110, and an etched secondshielding layer 112. For example, a remaining portion of the firstshielding layer 110 is arranged between the first sidewall 408 and thesecond sidewall 410 is removed. As discussed above, both the phase shiftlayer 108 and the first shielding layer 110 may be etched using thesecond etchant. The second etchant may have a chemical compositionincluding fluorine and oxygen. Thus, the phase shift layer 108 is etchedsuch that portions of the phase shift layer 108 are removed whileportions underlying the sections of the first shielding layer 110 andthe second shielding layer 112 remain. In this manner, the secondetching step forms a plurality of phase shift sections in the phaseshift layer by etching through the first shielding layer.

Accordingly, in one embodiment, the phase shift layer 108, the firstshielding layer 110, and the second shielding layer 112 are etched intoa plurality of patterned photomask sections having aligned sidewalls.Due to the selection of materials the three layers: the phase shiftlayer 108, the first shielding layer 110, and the second shielding layer112 may be etched using two etching steps rather than each layerrequiring an individual etching step. Thus, the process of forming thepatterned photomask 104 can be simplified.

FIGS. 5-15 cross-sectional views of some embodiment of views of someembodiments of a photomask at various stages of patterning are provided.The structures disclosed in FIGS. 5-15 are not limited to a particularmethod, but instead may stand alone as structures independent of themethod.

At FIG. 5, a transparent layer 106 is received. The transparent layer106 may be, for example, quartz, carbide substrate, or a siliconsubstrate. The transparent layer 106 may have, for example, a thicknessof between about 6-7 millimeters (mm). In some embodiments, thetransparent layer has a thickness of approximately 6.35 mm. Thetransparent layer 106 acts a substrate for formation of the photomaskstack 302.

As illustrated by FIG. 6, a phase shift layer 108 is formed over thetransparent layer 106. The phase shift layer 108 is comprised of a phaseshift material. For example, the phase shift material may be an opaquelayer of molybdenum silicide (MoSi). As described above, the phase shiftlayer 108 is configured to allow only a small percentage of light topass. Accordingly, in some embodiments, the phase shift layer 108 is notformed as alternating layers of molybdenum and silicide, like a Braggreflector. Instead, the phase shift layer 108 is formed as an opaqueshielding layer of MoSi that is used during photolithographic exposuresto aid patterning of deep ultraviolet radiation. In other embodiments,the phase shift layer 108 comprises an opaque layer of molybdenumsilicide oxynitride (Mo_(x)Si_(y)ON_(z)), which is used as a half tonematerial in optical lithography.

As illustrated by FIG. 7, a first shielding layer 110 is formed over thephase shift layer 108. The first shielding layer 110 may have athickness in a range between about 18 nm to about 30 nm. In someembodiments, the first shielding layer 110 may be comprised of a firstmaterial, such as tantalum (Ta), for example, tantalum borate(B₅O₁₅Ta₃). The first material may be selected for having a high opticaldensity despite a thinner thickness. Alternatively or additionally, thefirst material may be selected based on its ability to be etched byparticular etchant.

As illustrated by FIG. 8, a second shielding layer 112 is formed overthe first shielding layer 110. The second shielding layer 112 may have athickness in a range between about 1 nm to about 10 nm. Accordingly, thefirst shielding layer 110 may have a greater thickness than the secondshielding layer 112. In some embodiments, the second shielding layer 112may be comprised of a second material. The second material may beselected based on an optical property of the second material at aspecific thickness. For example, the second material may be selectedbased on it optical density or reflectivity. In some embodiments, thesecond material may be chromium (Cr) or a Cr based material. In someembodiments, the second material is chromium oxide (CrO_(x)) or chromiumnitride (CrN_(x)).

As illustrated by FIG. 9, a photoresist 304 is formed over the secondshielding layer 112. The photoresist 304 may have a thickness in a rangebetween about 10 nm and about 100 nm. In some embodiments, thephotoresist 304 may comprise a chemically amplified resin (CAR). Thephotoresist 304 may comprise a positive tone photoresist, which becomessoluble when exposed to radiation. In other embodiments, the photoresist304 may comprise a negative tone photoresist, which becomes insolublewhen exposed to radiation.

As illustrated by FIG. 10, the photoresist 304 is etched intophotoresist sections 1010 through exposure. For example, the photoresist304 may be exposed using an electron beam (e-beam) writer. The exposedphotoresist material is subsequently developed to remove weaker sectionsof the exposed photoresist material. In some embodiments, the layer ofphotoresist may be exposed using an electron beam (e-beam) writer. Theexposed photoresist layer 304 is subsequently developed to remove weakersections of the exposed photoresist material.

As illustrated by FIG. 11, the first shielding layer 110 and the secondshielding layer 112 may be etched by a first etchant. In variousembodiments, the first etchant may comprise a dry etchant have anetching chemistry comprising a chlorine species (e.g., Cl₂O₂, etc.) andoxygen, or a wet etchant. The first etchant, like the first material andsecond material is selected such that it is able to etch both the firstmaterial and at least a portion of the second material. The firstetchant may be applied for a first predetermined time to etch the firstand second shielding layers.

The second shielding layer 112 is etched into a plurality of secondshielding sections 1120, and the first shielding layer 110 is at leastpartially etched into a plurality of first shielding sections 1130underlying the photoresist sections 1010. For example, the firstshielding layer 110 may be etched into a plurality of first shieldingsections 1130 having sidewalls that extend to a first depth, d₁, in thefirst shielding layer 110 that is less than the thickness, t₁, of thefirst shielding layer 110.

As illustrated by FIG. 12, the photoresist sections 1010 is strippedfrom the photomask stack.

As illustrated by FIG. 13, the first shielding layer 110 and the phaseshift layer 108 may be etched by a second etchant. In variousembodiments, the second etchant may comprise a dry etchant have anetching chemistry comprising a fluorine species (e.g., CF₄, CHF₃, C₄F₈,SF₆, etc.) and oxygen, or a wet etchant (e.g., vapor-phase hydrofluoricacid (vHF), hydrofluoric acid (HF), BOE, or Tetramethylammoniumhydroxide (TMAH)). The second etchant, like the first material and phaseshift material, may be selected such that it is able to etch both thefirst material and the phase shift material can be etched in singleprocess step. The second etchant may be applied for a secondpredetermined time to etch the second shielding layer, with the secondpredetermined time being the same or different from the firstpredetermined time.

The second etchant etches through the remainder of the first shieldinglayer 110 between the first shielding sections 1130 such that the firstshielding sections 1130 are separated by gaps. Accordingly, the portionsof the first shielding layer 110 between the first shielding sections1130 equal to the thickness, t₁ (shown in FIG. 11), of the firstshielding layer 110 minus the first depth, d₁ (shown in FIG. 11), areremoved. Likewise, the phase shift layer 108 is etched by the secondetchant, in that etching step, such that phase shift sections 1302underlie and are aligned with the first shielding sections 1130. Thesecond etchant stops of the transparent layer 106, such that uppersurface of the transparent layer 106 remains substantially planar orsubstantially in place.

As illustrated by FIG. 14, a protective layer 1402 is formed over atleast one of the second shielding sections 1120. Here, protectedsections 1404, 1406, and 1408 have the protective layer 1402.Unprotected sections 1410 and 1412 do not have the protective layer1402. In some embodiments, the protective layer 1402 may extend down thesidewalls of the second shielding sections 1120, first shieldingsections 1130, and phase shift sections 1302. Protective layer 1402 canbe photoresist, for example, or can be a protective nitride, oxide orother material that have been patterned and etched using a photomask(not shown).

As illustrated by FIG. 15, unprotected sections 1410 and 1412 are etchedto remove the first shielding sections 1130 and second shielding section1120, such that there are not layers overlying the phase shift sections1302 of the unprotected sections. Then the protective layer 1402 can beremoved from the protected sections 1404, 1406, and 1408.

With reference to FIG. 16, a flowchart 1600 of some embodiments of themethod of FIGS. 5-15 is provided.

At 1602, a transparent layer is received as a substrate. See, forexample, FIG. 5.

At 1604, a phase shift layer is formed over the transparent layer. Insome embodiments the phase shift layer is comprised of molybdenumsilicide (MoSi). See, for example, FIG. 6,

At 1606, a first shielding layer is formed over the phase shift layer.The first shielding layer is comprised of a first material. The firstmaterial may be selected for specific optical properties. For example, aspecific optical density at a particular thickness. See, for example,FIG. 7.

At 1608, a second shielding layer is formed over the first shieldinglayer. The second shielding layer is comprised of a second material. Thesecond material may be selected for specific optical properties, suchthat the first shielding layer and the second shielding layer have adesired optical property and/or physical characteristic, such as theability to be etched by the same etchant. The phase shift layer, thefirst shielding layer, and the second shielding layer form a photomaskstack. See, for example, FIG. 8.

At 1610, a photoresist is formed over the second shielding layer. See,for example, FIG. 9.

At 1612, the photoresist is processed. In some embodiments, thetransparent layer, photomask stack, and the photoresist layer are bakedin a post exposure bake. The photoresist is etched to form a pluralityof photoresist sections based on a desired pattern. See, for example,FIG. 10.

At 1614, the first shielding layer and the second shielding layer areetched with a first etchant to form a plurality of first shieldingsections. Each first shielding section has a first shielding firstsidewall and a first shielding second sidewall. A first photoresistsidewall is aligned with the first shielding first sidewall and a secondphotoresist sidewall is aligned with the first shielding secondsidewall. See, for example, FIG. 11.

At 1616, the photoresist is stripped once the first shielding layer andsecond shielding layer are etched. In some embodiments, this involvesstripping the photo resist sections. See, for example, FIG. 12.

At 1618, the first shielding layer and the phase shift layer are etchedwith a second etchant to form a plurality of second shielding sectionsaligned with the plurality of first shielding sections. See, forexample, FIG. 13.

At 1620, a protective layer is formed over at least some of the secondshielding layers. The protective layer may extend down the sidewalls ofthe second shielding sections, first shielding sections, and phase shiftsections. See, for example, FIG. 14.

At 1622, sections not covered by the protective layer are etched toremove the first shielding sections and the second sections, leaving thephase shift section. See, for example, FIG. 15.

Accordingly, the patterned photomask having a plurality of shieldinglayers is described. The materials of the first and second shieldinglayers is selected for desired optical properties and physicalcharacteristics of the combined first and second shielding layers. Forexample, if a first shielding layer is selected for an optical property,such as, absorbance, the second shielding layer may be selected for toimprove other optical properties of the combined first and secondshielding layer. In one embodiment, the second shielding layer may beselected so that the combined first and second shielding layers have adesired optical density or thickness. Furthermore, the materials for thefirst and second shielding layers may be selected so that both the firstand second shielding layer can be etched by a first etchant. Likewise,the material of the first shielding layer may be selected so that asecond etchant can etch both the first shielding layer and the phaseshift layer. Thus, the first shielding layer does not require anindividual etching step. In this manner, the process for creating thephotomask can be simplified.

In some embodiments, a photomask for mask patterning is described. Thephotomask includes a phase shift layer overlying a transparent layer.The photomask also includes a first shielding layer overlying the phaseshift layer. The first shielding layer has a first thickness and a firstoptical density. The photomask further includes a second shielding layeroverlying the first shielding layer. The second shielding layer has asecond thickness and a second optical density. The second thickness isless that than the first thickness and the second optical density isless than the first optical density.

In some embodiments, a photomask for mask patterning includes a phaseshift layer comprised of molybdenum silicide (MoSi) overlying atransparent layer comprised of quartz. The photomask further includes afirst shielding layer overlying the phase shift layer. The firstshielding layer is comprised of tantalum borate and has a firstthickness of approximately 20 nm to 28 nm. The photomask also includes asecond shielding layer overlying the first shielding layer. The secondshielding layer is comprised of chromium nitride and has a secondthickness of approximately 3 nm to 7 nm.

In some embodiments a method of forming a photomask is described. Themethod includes forming a phase shift layer comprised of molybdenumsilicide (MoSi) overlying a transparent layer comprised of quartz. Themethod also includes forming a first shielding layer overlying the phaseshift layer. The first shielding layer has a first thickness and a firstoptical density. The method also includes forming a second shieldinglayer overlying the first shielding layer. The second shielding layerhas a second thickness and a second optical density. The secondthickness is less that than the first thickness and the second opticaldensity is less than the first optical density. The method also includesforming a photoresist layer overlying the second shielding layer.

Although the disclosure has been shown and described with respect to acertain aspect or various aspects, equivalent alterations andmodifications will occur to others of ordinary skill in the art uponreading and understanding this specification and the annexed drawings.In particular regard to the various functions performed by the abovedescribed components (assemblies, devices, circuits, etc.), the terms(including a reference to a “means”) used to describe such componentsare intended to correspond, unless otherwise indicated, to any componentwhich performs the specified function of the described component (i.e.,that is functionally equivalent), even though not structurallyequivalent to the disclosed structure which performs the function in theherein illustrated exemplary embodiments of the disclosure. In addition,while a particular feature of the disclosure may have been disclosedwith respect to only one of several aspects of the disclosure, suchfeature may be combined with one or more other features of the otheraspects as may be desired and advantageous for any given or particularapplication. Furthermore, to the extent that the terms “including”,“includes”, “having”, “has”, “with”, or variants thereof are used ineither the detailed description and the claims, such terms are intendedto be inclusive in a manner similar to the term “comprising”.

What is claimed is:
 1. A photomask for mask patterning, comprising: atransparent layer comprising quartz; a phase shift layer comprisingmolybdenum silicide (MoSi) overlying the transparent layer; and an uppershielding layer overlying the phase shift layer, wherein the uppershielding layer comprises chromium and has a thickness of 3 nm to 7 nm.2. The photomask of claim 1, further comprising: a lower shielding layerseparating the phase shift layer from the upper shielding layer, whereinthe lower shielding layer has a material composition that differs fromthat of the upper shielding layer.
 3. The photomask of claim 2, whereinthe lower shielding layer has a first optical density and the uppershielding layer has a second optical density, wherein the second opticaldensity is less than the first optical density.
 4. The photomask ofclaim 3, wherein the first optical density is greater than 1 and thesecond optical density is less than
 1. 5. The photomask of claim 2,wherein the lower shielding layer has a first thickness and the uppershielding layer has a second thickness, the second thickness is lessthan the first thickness.
 6. The photomask of claim 2, wherein the lowershielding layer and the upper shielding layer have a combined opticaldensity that is at least 1.8.
 7. The photomask of claim 1, wherein thephase shift layer covers some portions of the transparent layer whileleaving other portions of the transparent layer uncovered.
 8. Thephotomask of claim 1, wherein the upper shielding layer covers someportions of the phase shift layer but leaves other portions of the phaseshift layer uncovered.
 9. The photomask of claim 1, wherein the uppershielding layer comprises chromium nitride.
 10. A photomask for maskpatterning, comprising: a transparent layer comprising quartz; a phaseshift layer overlying the transparent layer; a first shielding layeroverlying the phase shift layer, wherein the first shielding layer has afirst thickness; and a second shielding layer overlying the firstshielding layer and having a non-zero second thickness, wherein thesecond shielding layer comprises chromium and the non-zero secondthickness is 7 nm or less.
 11. The photomask of claim 10, wherein thefirst shielding layer comprises tantalum or boron.
 12. The photomask ofclaim 10, wherein the phase shift layer covers some portions of thetransparent layer while leaving other portions of the transparent layeruncovered.
 13. The photomask of claim 10, wherein the first shieldinglayer covers some portions of the phase shift layer but leaves otherportions of the phase shift layer uncovered.
 14. The photomask of claim10, wherein the first shielding layer and the second shielding layercollectively have a combined optical density greater than one.
 15. Thephotomask of claim 14, wherein the combined optical density isapproximately 1.8.
 16. The photomask of claim 11, wherein the firstshielding layer has a first optical density and the second shieldinglayer has a second optical density, and wherein the second opticaldensity is less than the first optical density.
 17. The photomask ofclaim 16, wherein the first optical density is approximately 1.6 and thesecond optical density is approximately 0.23.
 18. The photomask of claim16, wherein the first optical density is greater than 1 and the secondoptical density is less than
 1. 19. A photomask for mask patterning,comprising: a transparent layer comprising quartz; a phase shift layercomprising molybdenum silicide (MoSi) overlying the transparent layer; afirst shielding layer overlying the phase shift layer, wherein the firstshielding layer has a first thickness; and a second shielding layeroverlying the first shielding layer, wherein the second shielding layercomprises chromium and has a second thickness ranging between aboutone-tenth to about one-third of the first thickness.
 20. The photomaskof claim 19, wherein the first shielding layer has a first opticaldensity and the second shielding layer has a second optical density, andwherein the second optical density is less than the first opticaldensity.